This disclosure relates to devices and methods for measuring the size of a distant object, or feature thereof, in a field of view of an optical sighting device, such as a handheld rangefinder, riflescope, or binoculars.
Knowing a dimension of a distant object is essential for many people, such as game hunters, foresters, and property owners. For example, game hunters may not only need to ensure an animal's antlers or horns are of sufficient size to satisfy hunting regulations but they may also wish to determine a trophy quality of the animal before taking it. Similarly, foresters may need to determine a size of a stand of trees to estimate board feet. Likewise, a property owner may wish to estimate the amount of fencing material needed for a stretch of land without manually measuring the dimensions. However, accurately judging the size of the distant object, such as the trophy size of an animal, is difficult even for experienced hunters and guides.
Reticles used with riflescopes and rangefinders and laser surveying instruments are known to allow a user to estimate a range to a distant object and adjust an aiming point for elevation holds. For example, The Leatherwood Sporter™ sold by Hi-Lux, Inc., Torrance, Calif., USA, contains an Auto/Range Reticle that automatically compensates for bullet trajectory based on a range to the target. This reticle contains framing marks that allow the user to overlay, or frame, a known dimension at the target to achieve an elevation adjustment in the scope. By way of example, an average sized deer has a chest height of about 18 inches, deer and antelope are about 54 inches tall, a wild pig is about 48 inches long, and a coyote is about 36 inches long. When the target is located in the riflescope's field of view, the appropriate framing mark is overlaid on the target. For example, if the user is aiming at an adult buck, the user could overlay an 18 inch vertical framing mark on the chest cavity. The magnification of the scope is adjusted until the framing mark frames the dimension of the target. These steps automatically adjust the riflescope relative to the rifle's barrel to compensate bullet drop at the determined range. The user can align the center of the riflescope's crosshairs on the intended point of impact without further adjustment and fire the bullet.
Other known riflescopes having range-finding capability utilize a ratio of known dimensions to approximate range. For example, a riflescope could have reticle features, such as dots, cross-hair pairs, gaps, or posts, having known dimensions. When the reticle and riflescope are properly calibrated, the apparent size of a dot, for example, as viewed through the riflescope will subtend a known angle. By comparing the apparent size of the dot to an image of the target having a known height as viewed through the scope, the range to the target can be estimated. By way of example, the scope and reticle may be calibrated so that the dot covers a circular area having a six inch diameter at a range of 100 yards (i.e. the dot is subtending 5.7 MOA). If the dot covers only ⅔ of a target estimated to be 18 inches high (i.e. the dot covers a circular area having a 12 inch diameter at the unknown range), the estimated range can be calculated using the following formula: Estimated Range=(12 inches/6 inches)×100 yards=200 yards. As these examples have shown, the hunter must perform calculations and use estimations to determine the range to the distant target. This may not only yield inaccurate results, but may require the hunter to receive specialized training or carry additional equipment, such as a calculator or numerous conversion tables.
U.S. Pat. No. 5,291,262 of Dunne describes a sighting scope having means for measuring the apparent width of a target. The user aims the sighting scope at the target, determines the number of reticle markings between left and right edges of the target, and manually enters the number of reticle markings into a microcontroller using a keypad. The microcontroller then uses the number of markings, reticle marking data, and a target distance as determined by a laser rangefinder to calculate the actual width of the target.
In addition, U.S. Pat. No. 6,738,148 of Dunne et al. describes a device having a tree diameter function. First, the horizontal distance to the target tree stem is either entered manually using a keypad or determined by a laser rangefinder. Next, the user depresses increase or decrease buttons on the keypad to position visual brackets so that the brackets adjoin a visual image of a perceived stem diameter at a desired point on the tree stem. After the user depresses the accept button, a processor determines a vertical angle to the point on the tree, calculates the actual height and diameter of the tree at the selected point, and displays the calculated height and diameter on a housing-mounted display of the device. Dunne et al. also describes the device having an upper stem diameter function. The horizontal distance to the target tree stem is either entered manually using a keypad or determined by a laser rangefinder. Next, the user can enter a fixed width at the upper stem and the processor will automatically set visual brackets to the selected upper stem width for the horizontal distance to the target tree. The user can aim the device at the tree stem until the visual brackets are aligned with the left and right sides of the target tree and depress the accept button. After the processor calculates the height at the fixed width selected based upon the horizontal distance to the tree and angle of inclination, it displays the height on the housing mounted display of the device.
However, Dunne and Dunne et al. do not disclose an aiming mark to help aim the rangefinder at the target. In addition, the brackets are positioned at equal intervals which may obstruct the view of the target. Furthermore, the user must divert attention away from the target to the external keypad to enter data or position the visual brackets and the housing mounted display to view the height and/or diameter of the target.
The present inventors have recognized a need for improved devices and methods for measuring the size of a distant object in a field of view of an optical sighting device.